diff --git a/src/truetype/ttgxvar.c b/src/truetype/ttgxvar.c
index 370a92c7e..d2452f88c 100644
--- a/src/truetype/ttgxvar.c
+++ b/src/truetype/ttgxvar.c
@@ -42,6 +42,7 @@
 #include <ft2build.h>
 #include <freetype/internal/ftdebug.h>
 #include FT_CONFIG_CONFIG_H
+#include <freetype/internal/ftcalc.h>
 #include <freetype/internal/ftstream.h>
 #include <freetype/internal/sfnt.h>
 #include <freetype/tttags.h>
@@ -1207,7 +1208,7 @@
                   delta == 1 ? "" : "s",
                   vertical ? "VVAR" : "HVAR" ));
 
-      *avalue += delta;
+      *avalue = ADD_INT( *avalue, delta );
     }
 
   Exit: